The work in progress using avian species as experimental material seeks to clarify the precise origin of the cells of the mesencephalic nucleus of the trigeminal nerve by the method of heterotransplantation of cells from quail as biological markers and an ultrastructural characterization of features such as cellular morphology, relative number and distribution of cells within the nucleus. The changes observed in the cells of the nucleus at selected developmental stages will be studied in correlation with emergence of behavior (jaw movements or beak clapping) qualitatively and quantitatively under a) normal and b) experimental conditions in which an antithyroid drug is injected intravenously during embryonic stages. By using quail cells as natural biological markers, it has been possible to establish the origin of precursor cells of the mesencephalic nucleus from the embryonic mid brain level. The changes affecting the cells of the mesencephalic nucleus under normal conditions of development at ultrastructural levels are currently underway. BIBLIOGRAPHIC REFERNCES: Narayanan, Y. (1976) Localization of the avian mesencephalic nucleus of the trigeminal nerve by chorio-allantoic grafts. (Abstract) Paper to be presented at the Annual Meeting of the American Association of Anatomists to be held in Louisville, April 20-23, 1976. Murray A. Matthews and C.H. Narayanan (1976) Progressive changes in neural degenerative response to injury temporarily associated with postnatal maturation in thalamic relay nuclei. (Abstract) Paper presented at the Fifth Annual Joint Symposium of Louisiana and Texas Societies for Electron Microscopy, Feb. 5-8, 1976.